Transformer oil purification temperature requirements

The purification temperature of transformer oil (insulating oil) needs to balance dehydration/degassing efficiency and oil stability. The specific requirements are as follows:

1. Conventional purification temperature range

- Optimum temperature: 50~65℃

- Vacuum oil filter (main equipment): usually set at 55~65℃ (heater outlet oil temperature).

- Pressure oil filter: 40~60℃ (avoid high temperature damage to filter material).

Temperature selection basis

Temperature range:<50℃

Impact:Water evaporates slowly, vacuum degassing efficiency is low, especially the removal effect of dissolved water is poor.

Temperature range:| 50~65℃

Impact:Optimum balance point: water evaporates quickly (up to more than 90%), gas escapes, and oil oxidation is controllable. 

Temperature range:>70℃ 

Impact:Risk: accelerate oil oxidation (increase in acid value), damage antioxidant additives (such as T501). 

2. Special working condition adjustment

- High water content oil (such as water content>50ppm): can be temporarily heated to 65~70℃, but the processing time needs to be shortened (≤4 hours).

- High viscosity oil (such as old oil in cold areas): can be appropriately raised to 60~70℃ to reduce viscosity, but the acid value needs to be monitored.

- Nano oil filtration technology: low temperature operation (30~50℃) to avoid high temperature damage to the nano filter membrane.

3. Key restrictions

- Flash point limit: the purification temperature must be at least 20℃ lower than the oil flash point (such as the flash point of typical transformer oil ≥140℃, the temperature should be ≤120℃).

- Vacuum coordination: high temperature needs to be combined with high vacuum (such as ≤0.5kPa) to avoid local oil overheating.

- Hot oil circulation: when the transformer body is heated, the difference between the winding temperature and the oil temperature must be ≤10℃ (to prevent local overheating).

4. International standard reference

- IEC 60422:

- Oil index after purification: moisture ≤10ppm (transformer above 330kV), ≤15ppm (below 110kV);

- Withstand voltage ≥60kV (2.5mm gap).

- GB/T 7595: requires oil temperature ≤65℃ during purification, and acid value change ≤0.01mg KOH/g.

5. Operation suggestions

1. Pretreatment test: Determine the moisture, acid value, and dielectric loss (tanδ) in the oil to determine the heating intensity.

2. Step heating: First, circulate at low temperature (40℃) to remove free water, and then raise to 60℃ to remove dissolved water.

3. Real-time monitoring:

- Online moisture sensor (such as PPM detector);

- The temperature difference between the oil temperature in the vacuum tank and the outlet temperature of the heater must be ≤5℃ (to avoid local overheating).

6. Risk Warning

- Oil aging: Temperature > 70℃ for 2 hours may lead to an increase in oxidation products (such as furfural).

- Equipment safety: Electric heaters must be equipped with over-temperature protection (such as bimetallic switches).